What should you write down during a classroom walkthrough? It's the wrong question. Asking it forces you to observe and categorize at once, and the categorizing wins. Capture richly, ask what am I seeing, and let the rubric translation come later.
What to look for in the first five minutes of a classroom walkthrough: not the teacher, the learning. What the students are doing, what the room reveals, what the teacher's decisions show, and what the lesson says about the weeks before it.